1. Field of the Invention
The present invention relates generally to acoustic noise suppression systems, and, more particularly, to an improved method and means for suppressing environmental background noise from speech signals to obtain speech quality enhancement.
2. Description of the Prior Art
Acoustic noise suppression systems generally serve the purpose of improving the overall quality of the desired signal by distinguishing the signal from the ambient background noise. More specifically, in speech communications systems, it is highly desirable to improve the signal-to-noise ratio (SNR) of the voice signal to enhance the quality of speech. This speech enhancement process is particularly necessary in environments having abnormally high levels of ambient background noise, such as an aircraft, a moving vehicle, or a noisy factory.
A typical application for noise suppression is in a hearing aid. Environmental background noise is not only annoying to the hearing-impaired, but often interferes with their ability to understand speech. One method of addressing this problem may be found in U.S. Pat. No. 4,461,025, entitled "Automatic Background Noise Suppressor." According to this approach, the speech signal is enhanced by automatically suppressing the audio signal in the absence of speech, and increasing the audio system gain when speech is present. This variation of an automatic gain control (AGC) circuit examines the incoming audio waveform itself to determine if the desired speech component is present.
A second method for enhancing the intelligiblity of speech in a hearing aid application is described in U.S. Pat. No. 4,454,609. This technique emphasizes the spectral content of consonant sounds of speech to equalize the intensity of consonant sounds with that of vowel sounds. The estimated spectral shape of the input speech is used to modify the spectral shape of the actual speech signal so as to produce an enhanced output speech signal. For example, a control signal may select one of a plurality of different filters having particularized frequency responses for modifying the spectral shape of the input speech signal, thereby producing an enhanced consonant output signal.
A more sophisticated approach to a noise suppression system implementation is the spectral subtraction--or spectral gain modification--technique. Using this approach, the audio input signal spectrum is divided into individual spectral bands by a bank of bandpass filters, and particular spectral bands are attenuated according to their noise energy content. A spectral subtraction noise suppression prefilter is described in R. J. McAulay and M. L. Malpass, "Speech Enhancement Using a Soft-Decision Noise Suppression Filter," IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-28, no. 2, (April 1980), pp. 137-145. This prefilter utilizes an estimate of the background noise power spectral density to generate the speech SNR, which, in turn, is used to compute a gain factor for each individual channel. The gain factor is used as a pointer for a look-up table to determine the attenuation for that particular spectral band. The channels are then attenuated and recombined to produce the noise-suppressed output waveform.
However, in specialized applications involving relatively high background noise environments, an effective noise suppression technique is being sought. For example, some cellular mobile radio telephone systems currently offer a vehicle speakerphone option providing hands-free operation for the automobile driver. The mobile hands-free microphone is typically located at a greater distance from the user, such as being mounted overhead on the visor. The more distant microphone delivers a much poorer signal-to-noise level to the land-end party due to road and wind noise within the vehicle. Although the received speech at the land end is usually intelligible, the high background noise level can be very annoying.
Although the aforementioned prior art techniques may perform sufficiently well under nominal background noise conditions, the performance of these approaches becomes severely limited when used under such high background noise conditions. Utilizing typical noise suppression systems, the noise level over most of the audio band can be reduced by 10 dB without seriously affecting the voice quality. However, when these prior art techniques are used in relatively high background noise environments requiring noise suppression levels approaching 20 dB, there is a substantial degradation in voice quality.
A need, therefore, exists for an improved acoustic noise suppression system which provides sufficient background noise attenuation in high ambient noise environments without significantly affecting the quality of the desired signal.